am using apple 3gs 8gb. mobile switches off often even with complete charge

Hi siva90,

Welcome to the Apple Support Communities! I'm sorry to hear you are having these issues with your iPhone. If you are having issues with your iPhone unexpectedly shutting off or restarting, you may find the troubleshooting steps outlined in the following article helpful, up to and including contacting Apple Support directly:

If your iOS device restarts or displays the Apple logo unexpectedly - Apple Support

Regards

The battery has probably reached the end of its usable life. Make an appointment at the genius bar of your local Apple Store and see if you can still get it replaced in a phone that old, or buy a new phone.